Crustal Recycling Research Articles

Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains

Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics. Consideration of this requires understanding of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4, it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus limiting the time range over which they can be detected. Nonetheless, this might not be the case for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial 206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and even accurately dated, no matter how long they have dwelled in the mantle.

Long‐Lived Source Heterogeneities in the Galapagos Mantle Plume

AbstractMantle plume‐derived melts provide a unique glimpse into the chemical heterogeneity of the deep mantle. However, the exact processes involved in forming and sampling lower mantle heterogeneities remain unresolved and thus, require further investigation to understand the cumulative effects of planetary differentiation, crustal recycling, and mantle mixing that lead to their formation. The Galapagos Plume is exceptional in that its lavas sample four distinct isotopic mantle components. However, the origin of each of these end‐members remains debated. In this study, we investigate the spatial and temporal appearance and evolution of these four isotopic end‐members that comprise the present‐day Archipelago by examining ancient Galapagos lava flows preserved in the accreted terranes in Costa Rica and Panama. We discovered that the spatial relationship between each of the classically defined Galapagos Domains has remained relatively constant throughout the evolution of the plume. Our new results extend the time scale of distinct isotopic heterogeneity in the source of the plume to at least 70 Ma for the Southern Domain and 90 Ma for the Central, and Northern Domains, suggesting that geochemically heterogeneous plumes rising through the mantle preserve distinct isotopic heterogeneity on the time scale of tens of millions of years. Additionally, trace element systematics of olivine crystals from the Galapagos‐related Quepos and Azuero terranes (50–70 ma) suggest derivation from sources that included recycled components. Collectively, these results suggest that mantle plumes not only sample large‐scale heterogeneities within the deep mantle, but that these source heterogeneities can remain sampled throughout the lifetime of a plume.

Paleomagnetism of IODP Site U1380: Implications for the Forearc Deformation in the Costa Rican Erosive Convergent Margin

The destructive nature of subduction erosion poses challenges to fully understanding the evolution of erosive convergent margins that are critical to understanding crustal recycling and seismogenesis. Forearc deformation holds important clues to the evolution of erosive convergent margins. Here we present detailed paleomagnetic and structural analyses of IODP Site U1380 cores from the middle slope of the forearc of the Costa Rican erosive convergent margin. The analyses reveal a strong deformation zone from ~490 to ~550 mbsf that is characterized by abundant fissility/foliations shallower than the bedding. Similar relatively strong deformation zones are recognized from the frontal prism and upper slope sites, and are broadly correlative, forming a zone of strong deformation across the forearc. This zone spans ~2.0 to 1.83 Ma and the deformation likely occurred briefly at ~1.80 Ma. The widespread, short-lived, and strong deformation is interpreted as a result of intense subhorizontal shear following the rapid forearc subsidence driven by the dramatic subduction erosion associated with the abrupt onset of the Cocos Ridge subduction. Given the typical occurrence of forearc subsidence by subduction erosion, similar styles of deformation are probably common in other erosive convergent margins as well.

Water and Volatile Inventories of Mercury, Venus, the Moon, and Mars

We review the geochemical observations of water, $\mbox{D}/\mbox{H}$ and volatile element abundances of the inner Solar System bodies, Mercury, Venus, the Moon, and Mars. We focus primarily on the inventories of water in these bodies, but also consider other volatiles when they can inform us about water. For Mercury, we have no data for internal water, but the reducing nature of the surface of Mercury would suggest that some hydrogen may be retained in its core. We evaluate the current knowledge and understanding of venusian water and volatiles and conclude that the venusian mantle was likely endowed with as much water as Earth of which it retains a small but non-negligible fraction. Estimates of the abundance of the Moon’s internal water vary from Earth-like to one to two orders of magnitude more depleted. Cl, K, and Zn isotope anomalies for lunar samples argue that the giant impact left a unique geochemical fingerprint on the Moon, but not the Earth. For Mars, an early magma ocean likely generated a thick crust; this combined with a lack of crustal recycling mechanisms would have led to early isolation of the Martian mantle from later delivery of water and volatiles from surface reservoirs or late accretion. The abundance estimates of Martian mantle water are similar to those of the terrestrial mantle, suggesting some similarities in the water and volatile inventories for the terrestrial planets and the Moon.

Binary sources of Chinese loess as revealed by trace and REE element ratios

Chinese loess is generally accepted to be one of the most important archives for establishing the average geochemical compositions of the upper continental crust (UCC), assuming numerous upper crustal recycling processes. However, the effect of such recycling processes on elemental fractionation has still not been systemically and quantitatively assessed. This study investigates the trace and rare earth elements (REE) of the <75 μm and <5 μm silicate fractions of ten deserts and sandy lands in northern China, and of Chinese loess, to evaluate the effect of grain size on element ratios. Our results indicated that REE- and some trace elements (Y, Ta and Nb)-related ratios have the negligible effect of grain size, and can therefore be regarded as size-independent proxies to trace the source of eolian deposits. The principal component analysis (PCA) of these size-independent ratios was conducted for the deserts and loess samples. The PCA results reveal that Chinese loess was ultimately derived from the materials eroded from two tectonic settings, i.e. the northern Tibetan Plateau (NTP) and the Central Asian Orogenic Belt (CAO). We propose that the UCC component of Chinese loess can not only be ascribed to numerous upper crustal recycling processes, but also to the thorough mixing of materials from different tectonic settings.

Olivine Oxygen Isotope Evidence for Intracontinental Recycling of Delaminated Continental Crust

AbstractThis paper reports the oxygen isotope compositions of olivine from dunite, wehrlite, harzburgite, and lherzolite xenoliths entrained by the Early Cretaceous high‐Mg diorites and Late Cretaceous basalts in the central and eastern North China Craton, with the aim of constraining processes that happened in the lithospheric mantle. Olivine oxygen isotope compositions were analyzed in situ using secondary ion mass spectrometry. Olivine δ18O of harzburgite and lherzolite xenoliths hosted in the Fushan and Tietonggou diorites with Archean and Paleoproterozoic rhenium‐depletion model ages ranges from 5.1‰ to 5.8‰, similar to olivine from spinel lherzolite xenoliths in the Late Cretaceous Fuxin basalts (olivine δ18O = 5.2–5.4‰) and typical mantle olivine. In contrast, olivine δ18O from dunite and wehrlite xenoliths ranges from 6.1‰ to 7.7‰, significantly higher than the typical mantle values. The olivine oxygen isotope data, combined with the petrologic observations, mineral compositions, and whole‐rock trace element and Sr‐Nd‐Os isotope compositions of the peridotite, suggest that the peridotites in Early Cretaceous high‐Mg diorites had been affected by interaction with melt derived from delaminated continental crust. These data provide compelling evidence for the recycling of continental crust in an intracontinental setting.

Multiple sources for Archean granitoids in the Yalgoo area, Yilgarn Craton, Western Australia: Geochemical and isotopic evidence

In the Yalgoo area of the north-central Yilgarn Craton, three small granitoid bodies, named the Lady Lydia (LLG), Yalgoo east (YEG) and Yalgoo north granitoids (YNG) intruded ∼2.8 Ga mafic supracrustal rocks. They form two suites: i) granodiorites (from both the LLG and YEG) with strongly fractionated REE patterns, high LREE contents, low HREE contents, weakly positive Eu anomalies, and of Archean TTG affinity, and ii) biotite monzogranites (from the YNG) with very high LREE contents and prominent negative Eu anomalies. All the granitoids are calc-alkaline, with the granodiorites recording ages of 2763–2752 Ma, and the biotite monzogranites recording ages of 2636–2634 Ma. The REE and HFSE characteristics of the granodiorites favor an Archean subduction environment. Abundant xenocrystic zircons (3.00–2.94 Ga) in the granodiorites and their Lu-Hf isotopic characteristics imply a long history of crustal recycling in the north-central Yilgarn. The Lu-Hf isotopes indicate that the ∼2.76 Ga granodiorites and xenocrysts were formed from a composite source ranging in age from 3.5 to 3.0 Ga. Increased crustal thickening due to convergence (after cessation of subduction) caused melting in the upper/mid-crustal levels, with generation of parental melt at ∼2.64 Ga to source the biotite monzogranites. The latter are characterized by strongly −ve εHf(t) (−10 to −12) and largely Eoarchean TDM2(Hf) model ages (∼3.85–3.6 Ga). Significantly, they provide evidence of Eoarchean crustal material in this part of the Yilgarn Craton.

Magmatic inheritance vs. UHT metamorphism: Zircon petrochronology of granulites and petrogenesis of charnockitic leucosomes of the Socorro–Guaxupé nappe, SE Brazil

We present a detailed study of zircon petrochronology of UHT granulites combined with charnockite petrogenesis, using geochemical and SrNd isotopic constraints for the Socorro-Guaxupé Nappe (SGN) in southeast Brazil. This multi-technique approach is used to refine the timescales of pre-collisional arc magmatism and syn-collisional UHT metamorphism and to constrain the P-T-t evolution of granulites. The SGN features the transition from a subduction-related to a continental-collision setting, involving the formation of in-situ orthopyroxene- and hornblende-bearing leucosome. The application of the REE array plot approach enabled a clear distinction between pre-collisional (prior to garnet growth) and syn-collisional (during or after garnet growth) zircon crystallization events in the SGN. A period of pre-collisional arc magmatism at ca. 730–640 Ma is identified. Al-in-orthopyroxene thermometry yields minimum temperatures of ca. 900 °C, 12 kbar in the garnet-rich granulite, consistent with UHT metamorphism, followed by cooling with re-equilibration to ca. 770 °C at 10 kbar. Garnet growth near peak conditions prior to zircon, and small-scale equilibrium between later garnet cores and ca. 615 Ma zircon rims are revealed by the array plot approach. Zircon growth during melt crystallization is documented in well-developed sector- and oscillatory-zoned overgrowths and “soccer ball” zircon at ca. 613 Ma and 608 Ma. Additionally, the origin and sources of both granulites and leucosomes are investigated. Orthopyroxene- and hornblende-bearing leucosomes are present both as fractionated melts and cumulates. Leucosomes and their granulite residues belong to the shoshonitic series, characterized by high K2O contents, and they display negative ƐNd(t) values (−11.7 to −5.1) and high initial 87Sr/86Sr ratios (0.7055–0.7080). The combination of geochemical and isotopic evidence, in addition to zircon inheritance patterns, point to enriched-mantle sources in their origin rather than crustal recycling involving either Archean or Paleoproterozoic older crust. The enrichment of subcontinental lithospheric mantle is interpreted to be related to the subduction stage prior to collision.

Rapid crustal growth and efficient crustal recycling in the early Earth: Implications for Hadean and Archean geodynamics

The geodynamic regime of the early Earth remains elusive, with so far proposed hypotheses ranging from stagnant lid convection to rapid plate tectonics. Available geological data are severely limited for the first two billion years of the Earth's history, and this scarcity of relevant data is often compounded by the nonuniqueness of interpretation. Here we propose that the samarium–neodymium isotope evolution, which has been suggested to be consistent with stagnant lid convection in the early Earth, may be better understood as the result of rapid crustal growth and extensive crustal recycling. We delineate the permissible scenario of crustal evolution through geochemical box modeling with a Monte Carlo sampling of the model parameter space, and our results suggest that the net growth of continental crust was complete by the end of the Hadean and that the rate of crustal recycling could have been as high as 2−4×1022 kg Gyr−1 at that time and has gradually decreased since then. Such crustal evolution yields a specific prediction for the present-day distribution of crustal formation ages, which is shown to be in remarkable agreement with a recent estimate based on the global compilation of zircon age data. The mode of subsolidus mantle convection after the putative magma ocean is probably plate tectonics, but its style could have been very different from that of contemporary plate tectonics, characterized by more voluminous magmatism and more destructive subduction.

Resolving the crustal composition paradox by 3.8 billion years of slab failure magmatism and collisional recycling of continental crust

In the standard paradigm, continental crust is formed mainly by arc magmatism, but because the compositions of magma rising from the mantle are basaltic and continental crust is estimated to contain about 60% SiO2 and much less MgO than basalt, the two do not match. To resolve this paradox, most researchers argue that large amounts of magmatic fractionation produce residual cumulates at the base of the crust, which because arcs are inferred to have magmatically thickened crust, form eclogites that ultimately founder and sink into the mantle. Not only are there problems with the contrasting bulk compositions, but the standard model also fails because prior to collision most modern arcs do not have thick crust, as documented by their eruption close to sea level, and in cases of ancient arc sequences, their intercalation with marine sedimentary rocks.Our study of Cretaceous batholiths in the North American Cordillera resolves the crustal composition paradox because we find that most are not arc-derived as commonly believed; but instead formed during the waning stages of collision and consequent slab failure. Because the batholiths typically have silica contents >60% and are derived directly from the mantle, we argue that they are the missing link in the formation of continental crust.Slab failure magmas worldwide are compositionally similar to tonalite-trondhjemite-granodiorite suites as old as 3.8 Ga, which points to their collective formation by slab failure and long-lived plate tectonics. Our model also provides (1) an alternative solution to interpret compiled detrital zircon arrays, because episodic peaks that coincide with periods of supercontinent amalgamation are easily interpreted to represent collisions with formation of new crust by slab failure; and (2) that models of early whole-earth differentiation are more reasonable than those invoking progressive growth of continental crust.

A hafnium isotopic record of magmatic arcs and continental growth in the Iapetus Ocean: The contrasting evolution of Ganderia and the peri-Laurentian margin

We test the sensitivity of combined U-Pb-Hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens, using Ganderia from the Canadian Appalachian orogen as an example. Ganderia hosted a long-lived magmatic arc that began at least by 650Ma and continued until 450Ma, during which time it drifted northward from the Amazonian margin. Its isotopic record suggests arc activity began earlier, possibly at ~750Ma (proto-Ganderia) within an oceanic domain. The U-Pb-Hf zircon isotopic array suggests that Ganderia formed on a sliver of Paleoproterozoic-Mesoproterozoic basement (TDMc=2.2–1.1Ga). A transition towards more evolved Hf isotopic compositions between 650 and 600Ma is interpreted to coincide with its accretion to the Amazonian margin of Gondwana, consistent with other geological evidence. Accretion was followed by increasing amounts of juvenile crustal inputs between ~550 and 500Ma, which is interpreted to reflect subduction roll-back and the onset of drift from the Gondwanan margin. Trench retreat culminated in the separation of the Ganderia arc and formation of the exclusively oceanic ~510–450Ma Penobscot-Popelogan-Victoria arcs on Ganderia. Accretion of Ganderia to the active Notre Dame Subzone along the Laurentian margin occurred in the late Ordovician and is marked by the abrupt termination of Ganderian arc magmatism.The Notre Dame arc evolved along the Laurentian margin between ~515 and 425Ma and was built on Paleoproterozoic-Archean (T(DM)c=2.9–2.5Ga) crust. Supra-subduction zone magmatism occurred at ~490–460Ma during the closure of the Taconic Seaway and was dominated by the recycling of Laurentian crust, with progressive mixing of evolved and juvenile arc magmas over time. The preservation of very evolved Notre Dame arc zircon in Ganderian overstep sequences confirms the arrival of the leading edge of Ganderia to Laurentia by ~450Ma. The hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints, and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems.

Episodic crustal growth in the Tanzania Craton: evidence from Nd isotope compositions

ABSTRACT The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crust-mantle separation ages, and events marking juvenile crustal addition and crustal recycling. Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton, although a rock record of such antiquity is yet to be found there. The most significant period of juvenile crustal addition as well as crustal recycling is 2.7–2.6 Ga. The Nd isotopes of mafic samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton, with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle. The Nd isotope section reveals significant differences in Nd isotopes between the north craton and central craton; compared to the north craton, the central craton yields a Nd model age that is approximately 100 Ma older, and its eNd(t) values are more negative, indicating that the two parts of the craton have different mantle source regions. Different types of granitoids are distributed in the Tanzania Craton, such as high-K and low-Al granite, calc-alkaline granite, peraluminous granite and transitional types of tonalite-trondhjemite-granodiorites (TTGs). Most of the granitoids formed later than the mafic rocks in syn-collision and post-collision events.

Cold plumes trigger contamination of oceanic mantle wedges with continental crust-derived sediments: Evidence from chromitite zircon grains of eastern Cuban ophiolites

The origin of zircon grains, and other exotic minerals of typical crustal origin, in mantle-hosted ophiolitic chromitites are hotly debated. We report a population of zircon grains with ages ranging from Cretaceous (99 Ma) to Neoarchean (2750 Ma), separated from massive chromitite bodies hosted in the mantle section of the supra-subduction (SSZ)-type Mayarí-Baracoa Ophiolitic Belt in eastern Cuba. Most analyzed zircon grains (n = 20, 287 ± 3 Ma to 2750 ± 60 Ma) are older than the early Cretaceous age of the ophiolite body, show negative εHf(t) (−26 to −0.6) and occasional inclusions of quartz, K-feldspar, biotite, and apatite that indicate derivation from a granitic continental crust. In contrast, 5 mainly rounded zircon grains (297 ± 5 Ma to 2126 ± 27 Ma) show positive εHf(t) (+0.7 to +13.5) and occasional apatite inclusions, suggesting their possible crystallization from melts derived from juvenile (mantle) sources. Interestingly, younger zircon grains are mainly euhedral to subhedral crystals, whereas older zircon grains are predominantly rounded grains. A comparison of the ages and Hf isotopic compositions of the zircon grains with those of nearby exposed crustal terranes suggest that chromitite zircon grains are similar to those reported from terranes of Mexico and northern South America. Hence, chromitite zircon grains are interpreted as sedimentary-derived xenocrystic grains that were delivered into the mantle wedge beneath the Greater Antilles intra-oceanic volcanic arc by metasomatic fluids/melts during subduction processes. Thus, continental crust recycling by subduction could explain all populations of old xenocrystic zircon in Cretaceous mantle-hosted chromitites from eastern Cuba ophiolite. We integrate the results of this study with petrological-thermomechanical modeling and existing geodynamic models to propose that ancient zircon xenocrysts, with a wide spectrum of ages and Hf isotopic compositions, can be transferred to the mantle wedge above subducting slabs by cold plumes.

Age, provenance and Precambrian evolution of the Anabar shield from U-Pb and Lu-Hf isotope data on detrital zircons, and the history of the northern and central Siberian craton

The Anabar shield in northern Siberia is one of the world’s least studied Precambrian areas, and provides a ‘window’ into the crustal basement of the central and northern Siberian craton. We report U-Pb and Hf isotope data for detrital zircons sampled in a profile across its major structural units. They define a U-Pb age range from 1.8 to 3.4Ga with three main periods: 1.8–2.0Ga, 2.4–2.8Ga and 3.0–3.4 Ga. The oldest zircons yield super-chondritic εHf(t) implying that the parental magmas of their source rocks were juvenile, i.e. formed from depleted mantle (DM). Thus, the crustal basement of the Anabar shield, and probably the whole central and northern Siberian craton, started to form in the mid-Paleoarchean, and included no recycled crust. Zircons with 2.5–2.7Ga ages define two εHf(t) intervals. One is super-chondritic (+2 to +7) implying juvenile sources, the other is sub-chondritic (−3 to −12) indicative of recycled crust, probably formed at 3.2–3.4Ga, in magma sources. Nearly all 1.8–2.0Ga zircons have sub-chondritic εHf(t) (−2 to −29) implying derivation from sources dominated by recycled crust formed at ∼2.6Ga and ∼3.4Ga and little or no juvenile addition. These events accompanied amalgamation of the entire craton by welding of Archean domains.The Bekelekh unit of the Daldyn series has the highest proportion of ∼2.6Ga zircons and may be the oldest ‘nucleus’ of the Anabar shield, whereas the Kilegur unit of the same series is essentially Proterozoic (1.95Ga). The largest amount of 3.1–3.4Ga zircons, as well as common 2.6–2.7Ga zircons, occur in the Ambardakh unit of the Upper Anabar series. Our data suggest alternation of areas with dominant ages of 1.95Ga and ∼2.6Ga, with the younger zircons coming from granites and granulites, and the older ones from gneisses. They show no evidence for significant ages differences for the Anabar and Olenek provinces. The final amalgamation of the entire Siberian craton by welding of Archean blocks, may have taken place at around 1954±6Ma.

Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth

The processes for crustal recycling during the Archaean are unclear. Numerical simulations suggest that dense lower crust would have peeled off into the mantle, leading to a rapid concentration of buoyant silicic rocks that formed the continents.

Large sulfur isotope fractionations in Martian sediments at Gale crater

Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO_2 volatilized from ten sediment samples acquired by NASA’s Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in ^(34)S. Measured values of δ^(34)S range from −47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods.

Calcium isotopic fractionation in mantle peridotites by melting and metasomatism and Ca isotope composition of the Bulk Silicate Earth

To better constrain the Ca isotopic composition of the Bulk Silicate Earth (BSE) and explore the Ca isotope fractionation in the mantle, we determined the Ca isotopic composition of 28 peridotite xenoliths from Mongolia, southern Siberia and the Siberian craton. The samples are divided in three chemical groups: (1) fertile, unmetasomatized lherzolites (3.7–4.7 wt.% Al2O3); (2) moderately melt-depleted peridotites (1.3–3.0 wt.% Al2O3) with no or very limited metasomatism (LREE-depleted cpx); (3) strongly metasomatized peridotites (LREE-enriched cpx and bulk rock) further divided in subgroups 3a (harzburgites, 0.1–1.0% Al2O3) and 3b (fertile lherzolites, 3.9–4.3% Al2O3). In Group 1, δ44/40Ca of fertile spinel and garnet peridotites, which experienced little or no melting and metasomatism, show a limited variation from 0.90 to 0.99‰ (relative to SRM 915a) and an average of 0.94 ± 0.05‰ (2SD, n=14), which defines the Ca isotopic composition of the BSE. In Group 2, the δ44/40Ca is the highest for three rocks with the lowest Al2O3, i.e. the greatest melt extraction degrees (average 1.06±0.04‰, i.e. ∼0.1‰ heavier than the BSE estimate). Simple modeling of modal melting shows that partial melting of the BSE with 103ln⁡αperidotite-melt ranging from 0.10 to 0.25 can explain the Group 2 data. By contrast, δ44/40Ca in eight out of nine metasomatized Group 3 peridotites are lower than the BSE estimate. The Group 3a harzburgites show the greatest δ44/40Ca variation range (0.25–0.96‰), with δ44/40Ca positively correlated with CaO and negatively correlated with Ce/Eu. Chemical evidence suggests that the residual, melt-depleted, low-Ca protoliths of the Group 3a harzburgites were metasomatized, likely by carbonate-rich melts/fluids. We argue that such fluids may have low (≤0.25‰) δ44/40Ca either because they contain recycled crustal components or because Ca isotopes, similar to trace elements and their ratios, may be fractionated by kinetic and/or chromatographic effects of melt percolation in the mantle. The δ44/40Ca in Group 3b lherzolites (0.83–0.89‰) are lower than in the BSE as well, but the effects of metasomatism on δ44/40Ca are smaller, possibly because of the high Ca contents in their protoliths and/or smaller δ44/40Ca differences between the protoliths and metasomatic agents. The BSE estimates based on fertile peridotites in this study fall in the δ44/40Ca ranges for oceanic and continental basalts, various meteorites (achondrites; carbonaceous, ordinary and enstatite chondrites), Mars, and the Moon. These results provide benchmarks for the application of Ca isotopes to planet formation, mantle evolution, and crustal recycling.

Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin

Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and δ18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026–0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy δ18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu∗, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like δ18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome.The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ∼3.2 to 4.9GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the – possibly oxidising – metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0Ga Bushveld event.

Origin of sulfur and crustal recycling of copper in polymetallic (Cu-Au-Co-Bi-U ± Ag) iron-oxide-dominated systems of the Great Bear Magmatic Zone, NWT, Canada

The Great Bear Magmatic Zone, in northwest Canada, contains numerous polymetallic mineral occurrences, prospects, and deposits of the iron oxide copper-gold deposit (IOCG) family. The mineralization is hosted by the Treasure Lake Group and igneous rocks of the Great Bear arc and was deposited concomitantly with the arc magmatism (ca. 1.88 to 1.87 Ga). In situ δ 34S (n = 48) and δ 65Cu (n = 79) analyses were carried out on ore-related sulfides from a number of these systems. The δ 34S values mainly vary between 0 and +5‰, consistent with derivation of sulfur from the mantle. Lower δ 34S values (−7.7 to +1.4‰) from the Sue-Dianne breccia may indicate SO2 disproportionation of a magmatic hydrothermal fluid. The δ 65Cu values vary between −1.2 and −0.3‰, and are lower than the igneous δ 65Cu range of values (0.0 ± 0.27‰). The S and Cu isotopic data are decoupled, which suggests that Cu (and possibly some S) was dissolved and remobilized from supracrustal rocks during early stages of alteration (e.g., sodic alteration) and then precipitated by lower temperature, more oxidizing fluids (e.g., Ca-Fe-K alteration). A limited fluid inclusion dataset and δ 13C and δ 18O values are also presented. The δ 18Ofluid values are consistent with a magmatic origin or a host-rock equilibrated meteoric water source, whereas the δ 13Cfluid values support a marine carbonate source. Combined, the S and Cu isotopic data indicate that while the emplacement of the Great Bear magmatic bodies may have driven fluid convection and may be the source of fluids and sulfur, metals such as Cu could have been recycled from crustal sources.

Crustal recycling and juvenile addition during lithospheric wrenching: The Pontivy-Rostrenen magmatic complex, Armorican Massif (France), Variscan belt

The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315Ma (between 316.7±2.5Ma and 310.3±4.7Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7±2.7Ma. The leucogranites (−4.8<εNd (T)<2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK>1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (−4.0<εNd (T)<−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1<A/CNK<1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2<εNd (T)<−2.2; no inherited zircon) is metaluminous (0.7<A/CNK<1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.